Method for determining bit size of rank indicator in wireless communication system, and device therefor

ABSTRACT

Disclosed in the present application is a method by which a terminal reports a rank indicator to a base station in a wireless access system. The rank indicator reporting method comprises the steps of: setting a plurality of channel status information—reference signal (CSI-RS) resources for one CSI process through an upper layer; selecting one from among the plurality of CSI-RS resources; and reporting, to the base station, an indicator indicating the selected one CSI-RS resource and the rank indicator, wherein a bit size of the rank indicator is determined on the basis of a maximum number of antenna ports for the plurality of CSI-RS resources.

This application is a 35 USC § 371 National Stage entry of International Application No. PCT/KR2016/010130 filed on Sep. 9, 2016, and claims priority to U.S. Provisional Application No. 62/217,025 filed on Sep. 11, 2015, all of which are hereby incorporated by reference in their entireties as if fully set forth herein.

TECHNICAL FIELD

The present invention relates to a wireless communication system, and more particularly, to a method of determining a bit size of a rank indicator in a wireless communication system and an apparatus therefor.

BACKGROUND ART

MIMO (multi-input multi-output) technology corresponds to a technology for increasing data transmission and reception efficiency using a plurality of transmission antennas and a plurality of reception antennas instead of using a single transmission antenna and a single reception antenna. If a single antenna is used, a receiving end receives data through a single antenna path. On the contrary, if multiple antennas are used, the receiving end receives data through several paths, thereby enhancing transmission speed and transmission capacity and increasing coverage.

A single-cell MIMO operation can be divided into a single user-MIMO (SU-MIMO) scheme that a single user equipment (UE) receives a downlink signal in a single cell and a multi user-MIMO (MU-MIMO) scheme that two or more UEs receive a downlink signal in a single cell.

Channel estimation corresponds to a procedure of restoring a received signal by compensating a distortion of the signal distorted by fading. In this case, the fading corresponds to a phenomenon of rapidly changing strength of a signal due to multi-path time delay in wireless communication system environment. In order to perform the channel estimation, it is necessary to have a reference signal known to both a transmitter and a receiver. The reference signal can be simply referred to as an RS (reference signal) or a pilot depending on a standard applied thereto.

A downlink reference signal corresponds to a pilot signal for coherently demodulating PDSCH (physical downlink shared channel), PCFICH (physical control format indicator channel), PHICH (physical hybrid indicator channel), PDCCH (physical downlink control channel) and the like. A downlink reference signal can be classified into a common reference signal (CRS) shared by all UEs within a cell and a dedicated reference signal (DRS) used for a specific UE only. Compared to a legacy communication system supporting 4 transmission antennas (e.g., a system according to LTE release 8 or 9 standard), a system including an extended antenna configuration (e.g., a system according to LTE-A standard supporting 8 transmission antennas) is considering DRS-based data demodulation to efficiently manage a reference signal and support an enhanced transmission scheme. In particular, in order to support data transmission through an extended antenna, it may be able to define a DRS for two or more layers. Since a DRS and data are precoded by a same precoder, it is able to easily estimate channel information, which is used for a receiving end to demodulate data, without separate precoding information.

Although a downlink receiving end is able to obtain precoded channel information on an extended antenna configuration through a DRS, it is required for the downlink receiving end to have a separate reference signal except the DRS to obtain channel information which is not precoded. Hence, it is able to define a reference signal for obtaining channel state information (CSI), i.e., a CSI-RS, at a receiving end in a system according to LTE-A standard.

DISCLOSURE OF THE INVENTION Technical Problem

Based on the aforementioned discussion, a method of determining a bit size of a rank indicator in a wireless communication system and an apparatus therefor are proposed in the following.

Technical tasks obtainable from the present invention are non-limited the above-mentioned technical task. And, other unmentioned technical tasks can be clearly understood from the following description by those having ordinary skill in the technical field to which the present invention pertains.

Technical Solution

To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, according to one embodiment, a method of reporting a rank indicator, which is reported to an eNB by a user equipment (UE) in a wireless access system, includes the steps of configuring a plurality of CSI-RS (channel status information—reference signal) resources for a single CSI (channel status information) process via a higher layer, selecting a single CSI-RS resource from among a plurality of the CSI-RS resources, and reporting an indicator indicating the selected single CSI-RS resource and the rank indicator to the eNB. In this case, a bit size of the rank indicator can be determined based on the maximum number of antenna ports for a plurality of the CSI-RS resources.

To further achieve these and other advantages and in accordance with the purpose of the present invention, according to a different embodiment, a user equipment (UE) in a wireless access system includes a wireless communication module and a processor, the processor configured to select a single CSI-RS resource from among a plurality of CSI-RS (channel status information—reference signal) resources for a CSI (channel status information) process configured via a higher layer, the processor configured to report an indicator indicating the selected single CSI-RS resource and a rank indicator to an eNB. In this case, a bit size of the rank indicator can be determined based on the maximum number of antenna ports for a plurality of the CSI-RS resources.

Preferably, the rank indicator is determined using the selected signal CSI-RS resource.

And, the bit size of the rank indicator is determined based on the maximum number of antenna ports for a plurality of the CSI-RS resources and the maximum number of layers capable of being supported by the UE.

More preferably, the UE can transmit information on the maximum number of layers capable of being supported by the UE to the eNB.

In particular, independent precoding can be applied to each of a plurality of the CSI-RS resources.

Advantageous Effects

According to embodiments of the present invention, a UE can efficiently determine and report a bit size of a rank indicator in a wireless communication system, i.e., a wireless communication system to which 3D MIMO is applied.

Effects obtainable from the present invention may be non-limited by the above mentioned effect. And, other unmentioned effects can be clearly understood from the following description by those having ordinary skill in the technical field to which the present invention pertains.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.

FIG. 1 is a diagram for a structure of a downlink radio frame;

FIG. 2 is a diagram for an example of a resource grid of a downlink slot;

FIG. 3 is a diagram for structure of a downlink subframe;

FIG. 4 is a diagram for structure of an uplink subframe;

FIG. 5 is a diagram for a configuration of a wireless communication system including a plurality of antennas;

FIG. 6 illustrates a 2D active antenna system having 64 antenna elements;

FIG. 7 illustrates a 3D-MIMO system utilizing 2D-AAS;

FIG. 8 is a diagram illustrating a relation between an antenna element and an antenna port in 2D AAS system to which massive MIMO is applied;

FIG. 9 illustrates an example of PUCCH CSI feedback according to an embodiment of the present invention;

FIG. 10 is a diagram illustrating a change of a bit size of an RI according to a BI in accordance with an embodiment of the present invention;

FIG. 11 is a flowchart illustrating a method of determining a bit size of a rank indicator according to an embodiment of the present invention;

FIG. 12 is a diagram for a configuration of a base station and a user equipment applicable to one embodiment of the present invention.

BEST MODE MODE FOR INVENTION

The embodiments described in the following correspond to combinations of elements and features of the present invention in prescribed forms. And, the respective elements or features may be considered as selective unless they are explicitly mentioned. Each of the elements or features can be implemented in a form failing to be combined with other elements or features. Moreover, it is able to implement an embodiment of the present invention by combining elements and/or features together in part. A sequence of operations explained for each embodiment of the present invention can be modified. Some configurations or features of one embodiment can be included in another embodiment or can be substituted for corresponding configurations or features of another embodiment.

In this specification, embodiments of the present invention are described centering on the data transmission/reception relations between a user equipment and an eNode B. In this case, the eNode B may correspond to a terminal node of a network directly performing communication with the user equipment. In this disclosure, a specific operation explained as performed by an eNode B may be performed by an upper node of the eNode B in some cases.

In particular, in a network constructed with a plurality of network nodes including an eNode B, it is apparent that various operations performed for communication with a user equipment can be performed by an eNode B or other networks except the eNode B. ‘eNode B (eNB)’ may be substituted with such a terminology as a fixed station, a Node B, a base station (BS), an access point (AP) and the like. A terminal may be substituted with such a terminology as a relay node (RN), a relay station (RS), and the like. And, a terminal may be substituted with such a terminology as a user equipment (UE), a mobile station (MS), a mobile subscriber station (MSS), and the like.

Specific terminologies used in the following description are provided to help understand the present invention and the use of the specific terminologies can be modified into a different form in a range of not deviating from the technical idea of the present invention.

Occasionally, to prevent the present invention from getting vaguer, structures and/or devices known to the public are skipped or can be represented as block diagrams centering on the core functions of the structures and/or devices. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

Embodiments of the present invention may be supported by the standard documents disclosed in at least one of wireless access systems including IEEE 802 system, 3GPP system, 3GPP LTE system, 3GPP LTE-A (LTE-Advanced) system and 3GPP2 system. In particular, the steps or parts, which are not explained to clearly reveal the technical idea of the present invention, in the embodiments of the present invention may be supported by the above documents. Moreover, all terminologies disclosed in this document may be supported by the above standard documents.

The following description of embodiments of the present invention may be usable for various wireless access systems including CDMA (code division multiple access), FDMA (frequency division multiple access), TDMA (time division multiple access), OFDMA (orthogonal frequency division multiple access), SC-FDMA (single carrier frequency division multiple access) and the like. CDMA can be implemented with such a radio technology as UTRA (universal terrestrial radio access), CDMA 2000 and the like. TDMA can be implemented with such a radio technology as GSM/GPRS/EDGE (Global System for Mobile communications)/General Packet Radio Service/Enhanced Data Rates for GSM Evolution). OFDMA can be implemented with such a radio technology as IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, E-UTRA (Evolved UTRA), etc. UTRA is a part of UMTS (Universal Mobile Telecommunications System). 3GPP (3rd Generation Partnership Project) LTE (long term evolution) is a part of E-UMTS (Evolved UMTS) that uses E-UTRA. The 3GPP LTE adopts OFDMA in downlink (hereinafter abbreviated DL) and SC-FDMA in uplink (hereinafter abbreviated UL). And, LTE-A (LTE-Advanced) is an evolved version of 3GPP LTE. WiMAX may be explained by IEEE 802.16e standard (e.g., WirelessMAN-OFDMA reference system) and advanced IEEE 802.16m standard (e.g., WirelessMAN-OFDMA advanced system). For clarity, the following description mainly concerns 3GPP LTE and LTE-A standards, by which the technical idea of the present invention may be non-limited.

A structure of a downlink radio frame is explained in the following with reference to FIG. 1.

Referring to FIG. 1, in a cellular OFDM radio packet communication system, uplink/downlink data packet transmission is performed in a unit of subframe, wherein one subframe is defined by a given time interval that includes a plurality of OFDM symbols. The 3GPP LTE standard supports a type 1 radio frame structure applicable to frequency division duplex (FDD) and a type 2 radio frame structure applicable to time division duplex (TDD).

FIG. 1 is a diagram illustrating a structure of a type 1 radio frame. The downlink radio frame includes 10 subframes, each of which includes two slots in a time domain. A time required to transmit one subframe will be referred to as a transmission time interval (TTI). For example, one subframe may have a length of 1 ms, and one slot may have a length of 0.5 ms. One slot includes a plurality of OFDM symbols in a time domain and a plurality of resource blocks (RB) in a frequency domain. Since the 3GPP LTE system uses OFDM in a downlink, OFDM symbols represent one symbol period. The OFDM symbol may be referred to as SC-FDMA symbol or symbol period. The resource block (RB) as a resource allocation unit may include a plurality of continuous subcarriers in one slot.

The number of OFDM symbols included in one slot may vary depending on a configuration of a cyclic prefix (CP). Examples of the CP include an extended CP and a normal CP. For example, if the OFDM symbols are configured by the normal CP, the number of OFDM symbols included in one slot may be 7. If the OFDM symbols are configured by the extended CP, since the length of one OFDM symbol is increased, the number of OFDM symbols included in one slot is smaller than that of OFDM symbols in case of the normal CP. For example, in case of the extended CP, the number of OFDM symbols included in one slot may be 6. If a channel state is unstable like the case where the user equipment moves at high speed, the extended CP may be used to reduce inter-symbol interference.

If the normal CP is used, since one slot includes seven OFDM symbols, one subframe includes 14 OFDM symbols. At this time, first two or three OFDM symbols of each subframe may be allocated to a physical downlink control channel (PDCCH), and the other OFDM symbols may be allocated to a physical downlink shared channel (PDSCH).

The aforementioned structure of a radio frame is just an example only. The number of subframes included in a radio frame, the number of slots included in a subframe and the number of symbols included in a slot may be modified in various ways.

FIG. 2 is a diagram for an example of a resource grid of a downlink slot. FIG. 2 shows a case that an OFDM symbol is configured by a normal CP. Referring to FIG. 2, a downlink slot includes a plurality of OFDM symbols in a time domain and a plurality of resource blocks in a frequency domain. In this case, although FIG. 2 illustrates that a downlink slot includes seven OFDM symbols and a resource block includes twelve subcarriers, by which the present invention may be non-limited. Each element on the resource grid will be referred to as a resource element (RE). For example, an RE a (k, l) may correspond to an RE positioned at a kth subcarrier and an lth OFDM symbol. In case of a normal CP, one resource block includes 12*7 resource elements (in case of an extended CP, one resource block includes 12*6 resource elements). Since a space between subcarriers corresponds to 15 kHz, one resource block includes about 180 kHz in frequency domain. NDL corresponds to the number of resource blocks included in a downlink slot. A value of the NDL can be determined according to a downlink transmission bandwidth scheduled by a base station.

FIG. 3 is a diagram illustrating a structure of a downlink subframe. Referring to FIG. 3, maximum three OFDM symbols located at the front of the first slot of a subframe correspond to a control region to which a control channel is allocated. The other OFDM symbols correspond to a data region to which a physical downlink shared channel (PDSCH) is allocated. A basic unit of transmission becomes one subframe. In particular, PDCCH and PDSCH are assigned over two slots. Examples of downlink control channels used in the 3GPP LTE system include a Physical Control Format Indicator Channel (PCFICH), a Physical Downlink Control Channel (PDCCH), and a Physical Hybrid ARQ Indicator Channel (PHICH). The PCFICH is transmitted from the first OFDM symbol of the subframe, and carries information on the number of OFDM symbols used for transmission of the control channel within the subframe. The PHICH carries HARQ ACK/NACK signals in response to uplink transmission. The control information transmitted through the PDCCH will be referred to as downlink control information (DCI). The DCI includes uplink or downlink scheduling information, uplink transmission (Tx) power control command for a random UE group and the like. The PDCCH may include transport format and resource allocation information of a downlink shared channel (DL-SCH), transport format and resource allocation information of an uplink shared channel (UL-SCH), paging information on a paging channel (PCH), system information on the DL-SCH, resource allocation information of upper layer control message such as random access response transmitted on the PDSCH, a set of transmission (Tx) power control commands of individual user equipments (UEs) within a random user equipment group, transmission (Tx) power control command, and activity indication information of voice over Internet protocol (VoIP). A plurality of PDCCHs may be transmitted within the control region. The user equipment may monitor the plurality of PDCCHs. The PDCCH is transmitted on aggregation of one or a plurality of continuous control channel elements (CCEs). The CCE is a logic allocation unit used to provide the PDCCH with a coding rate based on the status of a radio channel The CCE corresponds to a plurality of resource element groups (REGs). The format of the PDCCH and the number of available bits of the PDCCH are determined depending on a correlation between the number of CCEs and a coding rate provided by the CCE. The base station determines a PDCCH format depending on the DCI which will be transmitted to the user equipment, and attaches cyclic redundancy check (CRC) to the control information. The CRC is masked with an identifier (for example, radio network temporary identifier (RNTI)) depending on usage of the PDCCH or owner of the PDCCH. For example, if the PDCCH is for a specific user equipment, the CRC may be masked with cell-RNTI (C-RNTI) of the corresponding user equipment. If the PDCCH is for a paging message, the CRC may be masked with a paging identifier (for example, paging-RNTI (P-RNTI)). If the PDCCH is for system information (in more detail, system information block (SIB)), the CRC may be masked with system information RNTI (SI-RNTI). If the PDCCH is for a random access response, the CRC may be masked with a random access RNTI (RA-RNTI).

FIG. 4 is a diagram for structure of an uplink subframe. Referring to FIG. 4, a UL subframe may be divided into a control region and a data region in a frequency domain. A physical uplink control channel (PUCCH) including uplink control information is allocated to the control region and a physical uplink shared channel (PUSCH) including user data is allocated to the data region. In order to maintain single carrier property, a UE does not transmit the PUCCH and the PUSCH at the same time. The PUCCH for one UE is allocated to a resource block pair in a subframe. The resource blocks belonging to the resource block pair occupy a different subcarrier with respect to two slots. This is represented as the resource block pair allocated to the PUCCH is frequency-hopped at a slot boundary.

MIMO System Modeling

Hereinafter, a MIMO system will be described. MIMO refers to a method using multiple transmit antennas and multiple receive antennas to improve data transmission/reception efficiency. Namely, a plurality of antennas is used at a transmitter or a receiver of a wireless communication system so that capacity can be increased and performance can be improved. MIMO may also be referred to as multi-antenna in this disclosure.

MIMO technology does not depend on a single antenna path in order to receive a whole message. Instead, MIMO technology completes data by combining data fragments received via multiple antennas. The use of MIMO technology can increase data transmission rate within a cell area of a specific size or extend system coverage at a specific data transmission rate. MIMO technology can be widely used in mobile communication terminals and relay nodes. MIMO technology can overcome a limited transmission capacity encountered with the conventional single-antenna technology in mobile communication.

FIG. 5 illustrates the configuration of a typical MIMO communication system. A transmitter has N_(T) transmit (Tx) antennas and a receiver has N_(R) receive (Rx) antennas. Use of a plurality of antennas at both the transmitter and the receiver increases a theoretical channel transmission capacity, compared to the use of a plurality of antennas at only one of the transmitter and the receiver. Channel transmission capacity increases in proportion to the number of antennas. Therefore, transmission rate and frequency efficiency are increased. Given a maximum transmission rate R_(o) that may be achieved with a single antenna, the transmission rate may be increased, in theory, to the product of R_(o) and a transmission rate increase rate R_(i) in the case of multiple antennas, as indicated by Equation 1. R_(i) is the smaller of N_(T) and N_(R). R _(i)=min(N _(T) ,N _(R))   [Equation 1]

For example, a MIMO communication system with four Tx antennas and four Rx antennas may theoretically achieve a transmission rate four times that of a single antenna system. Since the theoretical capacity increase of the MIMO wireless communication system was verified in the mid-1990s, many techniques have been actively developed to increase data transmission rate in real implementations. Some of these techniques have already been reflected in various wireless communication standards including standards for 3rd generation (3G) mobile communications, next-generation wireless local area networks, etc.

Active research up to now related to MIMO technology has focused upon a number of different aspects, including research into information theory related to MIMO communication capacity calculation in various channel environments and in multiple access environments, research into wireless channel measurement and model derivation of MIMO systems, and research into space-time signal processing technologies for improving transmission reliability and transmission rate.

Communication in a MIMO system will be described in detail through mathematical modeling. It is assumed that N_(T) Tx antennas and N_(R) Rx antennas are present as illustrated in FIG. 5. Regarding a transmission signal, up to N_(T) pieces of information can be transmitted through the N_(T) Tx antennas, as expressed as the following vector. S=[S ₁ , S ₂ , . . . , S _(N) _(T) ]^(T)

Individual pieces of the transmission information S₁, S₂, . . . , S_(N) _(T) may have different transmit powers. If the individual transmit powers are denoted by P₁, P₂, . . . , P_(N) _(T) respectively, then the transmission power-controlled transmission information may be given as Ŝ=[Ŝ ₁ , Ŝ ₂ , . . . , Ŝ _(N) _(T) ]^(T) =[P ₁ s ₁i , P₂ s ₂ , . . . , P _(N) _(T) s _(N) _(T) ]^(T)   [Equation 3]

The transmission power-controlled transmission information vector Ŝ may be expressed below, using a diagonal matrix P of transmission power.

$\begin{matrix} {\overset{\hat{}}{s} = {{\begin{bmatrix} P_{1} & \; & \; & 0 \\ \; & P_{2} & \; & \; \\ \; & \; & \ddots & \; \\ 0 & \; & \; & P_{N_{T}} \end{bmatrix}\begin{bmatrix} s_{1} \\ s_{2} \\ \vdots \\ s_{N_{T}} \end{bmatrix}} = {Ps}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack \end{matrix}$

Meanwhile, N_(T) transmission signals x₁, x₂, . . . , x_(N) _(T) to be actually transmitted may be configured by multiplying the transmission power-controlled information vector Ŝ by a weight matrix W. The weight matrix W functions to appropriately distribute the transmission information to individual antennas according to transmission channel states, etc. The transmission signals x₁, x₂, . . . , x_(N) _(T) are represented as a vector X, which may be determined by Equation 5. Here, W_(ij) denotes a weight of an i-th Tx antenna and a j-th piece of information. W is referred to as a weight matrix or a precoding matrix.

                                 [Equation  5] $X = {\begin{bmatrix} x_{1} \\ x_{2} \\ \vdots \\ x_{i} \\ \vdots \\ x_{N_{T}} \end{bmatrix} = {{\begin{bmatrix} w_{11} & w_{12} & \ldots & w_{1N_{T}} \\ w_{21} & w_{22} & \ldots & w_{2N_{T}} \\ \; & \; & \ddots & \; \\ w_{i1} & w_{i2} & \ldots & w_{iN_{T}} \\ \vdots & \; & \ddots & \; \\ w_{N_{T}1} & w_{N_{T}2} & \ldots & w_{N_{T}N_{T}} \end{bmatrix}\begin{bmatrix} {\overset{\hat{}}{s}}_{1} \\ {\overset{\hat{}}{s}}_{2} \\ \vdots \\ {\overset{\hat{}}{s}}_{j} \\ \vdots \\ {\overset{\hat{}}{s}}_{N_{T}} \end{bmatrix}} = {{W\overset{\hat{}}{s}} = {WPs}}}}$

Generally, the physical meaning of the rank of a channel matrix is the maximum number of different pieces of information that can be transmitted on a given channel. Therefore, the rank of a channel matrix is defined as the smaller of the number of independent rows and the number of independent columns in the channel matrix. Accordingly, the rank of the channel matrix is not larger than the number of rows or columns of the channel matrix. The rank of the channel matrix H (rank(H)) is restricted as follows. rank(H)≤min(N _(T) , N _(R))   [Equation 6]

A different piece of information transmitted in MIMO is referred to as a transmission stream or stream. A stream may also be called a layer. It is thus concluded that the number of transmission streams is not larger than the rank of channels, i.e. the maximum number of different pieces of transmittable information. Thus, the channel matrix H is determined by # of streams≤rank(H)≤min(N _(T) , N _(R))   [Equation 7]

“# of streams” denotes the number of streams. It should be noted that one stream may be transmitted through one or more antennas.

One or more streams may be mapped to a plurality of antennas in many ways. This method may be described as follows depending on MIMO schemes. If one stream is transmitted through a plurality of antennas, this may be regarded as spatial diversity. When a plurality of streams is transmitted through a plurality of antennas, this may be spatial multiplexing. A hybrid scheme of spatial diversity and spatial multiplexing may be contemplated.

CSI Feedback

Hereinbelow, a description of channel state information (CSI) reporting will be given. In the current LTE standard, a MIMO transmission scheme is categorized into open-loop MIMO operated without CSI and closed-loop MIMO operated based on CSI. Especially, according to the closed-loop MIMO system, each of the eNB and the UE may be able to perform beamforming based on CSI in order to obtain multiplexing gain of MIMO antennas. To acquire CSI from the UE, the eNB transmits RSs to the UE and commands the UE to feed back CSI measured based on the RSs through a PUCCH or a PUSCH.

CSI is divided into three types of information: an RI, a PMI, and a CQI. First, RI is information on a channel rank as described above and indicates the number of streams that can be received via the same time-frequency resource. Since RI is determined by long-term fading of a channel, it may be generally fed back at a cycle longer than that of PMI or CQI.

Second, PMI is a value reflecting a spatial characteristic of a channel and indicates a precoding matrix index of the eNB preferred by the UE based on a metric of signal-to-interference plus noise ratio (SINR). Lastly, CQI is information indicating the strength of a channel and indicates a reception SINR obtainable when the eNB uses PMI.

An advanced system such as an LTE-A system considers additional multi-user diversity through multi-user MIMO (MU-MIMO). Due to interference between UEs multiplexed in an antenna domain in MU-MIMO, the accuracy of CSI may significantly affect interference with other multiplexed UEs as well as a UE that reports the CSI. Accordingly, more accurate CSI than in single-user MIMO (SU-MIMO) should be reported in MU-MIMO.

In this context, the LTE-A standard has determined to separately design a final PMI as a long-term and/or wideband PMI, W1, and a short-term and/or subband PMI, W2.

For example, a long-term covariance matrix of channels expressed as Equation 8 may be used for hierarchical codebook transformation that configures one final PMI with W1 and W2. W=norm(W1W2)   [Equation 8]

In Equation 8, W2 is a short-term PMI, which is a codeword of a codebook reflecting short-term channel information, W is a codeword of a final codebook, and norm(A) is a matrix obtained by normalizing each column of matrix A to 1.

Conventionally, the codewords W1 and W2 are given as Equation 9.

$\begin{matrix} {{{W\; 1(i)} = \begin{bmatrix} X_{i} & 0 \\ 0 & X_{i} \end{bmatrix}},{{where}\mspace{14mu} X_{i}\mspace{14mu}{is}\mspace{14mu}{{Nt}/2}\mspace{14mu}{by}\mspace{14mu} M\mspace{14mu}{{matrix}.}}} & \left\lbrack {{Equation}\mspace{14mu} 9} \right\rbrack \\ {{{W\; 2(j)} = {\overset{\overset{r\mspace{11mu}{columns}}{︷}}{\begin{bmatrix} e_{M}^{k} & e_{M}^{l} & \; & e_{M}^{m} \\ \; & \; & \ldots & \; \\ {\alpha_{j}e_{M}^{k}} & {\beta_{j}e_{M}^{l}} & \; & {\gamma_{j}e_{M}^{m}} \end{bmatrix}}\mspace{14mu}\left( {{{if}\mspace{14mu}{rank}} = r} \right)}},} & \; \\ {{{{where}\mspace{14mu} 1} \leq k},l,{m \leq {M\mspace{14mu}{and}\mspace{14mu} k}},l,{m\mspace{14mu}{are}\mspace{14mu}{{integer}.}}} & \; \end{matrix}$

where Nt is the number of Tx antennas, M is the number of columns of a matrix Xi, indicating that the matrix Xi includes a total of M candidate column vectors. eMk, eMl, and eMm denote k-th, 1-th, and m-th column vectors of the matrix Xi in which only k-th, 1-th, and m-th elements among M elements are 0 and the other elements are 0, respectively. α_(j), β_(j), and γ_(j) are complex values each having a unit norm and indicate that, when the k-th, l-th, and m-th column vectors of the matrix Xi are selected, phase rotation is applied to the column vectors. At this time, i is an integer greater than 0, denoting a PMI index indicating W1 and j is an integer greater than 0, denoting a PMI index indicating W2.

In Equation 9, the codewords are designed so as to reflect correlation characteristics between established channels, if cross-polarized antennas are densely arranged, for example, the distance between adjacent antennas is equal to or less than half a signal wavelength. The cross-polarized antennas may be divided into a horizontal antenna group and a vertical antenna group and the two antenna groups are co-located, each having the property of a uniform linear array (ULA) antenna.

Therefore, the correlations between antennas in each group have the same linear phase increment property and the correlation between the antenna groups is characterized by phase rotation. Since a codebook is quantized values of channels, it is necessary to design a codebook reflecting channel characteristics. For convenience of description, a rank-1 codeword designed in the above manner may be given as Equation 10.

$\begin{matrix} {{W\; 1(i)*W\; 2(j)} = \begin{bmatrix} {X_{i}(k)} \\ {\alpha_{j}{X_{i}(k)}} \end{bmatrix}} & \left\lbrack {{Equation}\mspace{14mu} 10} \right\rbrack \end{matrix}$

In Equation 10, a codeword is expressed as an N_(T)×1 vector where NT is the number of Tx antennas and the codeword is composed of an upper vector X_(i)(k) and a lower vector α_(j)X_(i)(k), representing the correlation characteristics of the horizontal and vertical antenna groups, respectively. X_(i)(k) is expressed as a vector having the linear phase increment property, reflecting the correlation characteristics between antennas in each antenna group. For example, a discrete Fourier transform (DFT) matrix may be used for X_(i)(k).

As mentioned in the foregoing description, channel state information (CSI) includes CQI, PMI, RI, and the like in LTE system. All or a part of the CQI, the PMI, and the RI is transmitted depending on a transmission mode of a UE. When the CSI is periodically transmitted, it is referred to as periodic reporting. When the CSI is transmitted upon the request of a base station, it is referred to as aperiodic reporting. In case of the aperiodic reporting, a request bit, which is included in uplink scheduling information transmitted by a base station, is transmitted to a UE. The UE forwards CSI to the base station via a data channel (PUSCH) in consideration of a transmission mode of the UE. In case of the periodic reporting, a period and an offset in the period are signaled in a unit of a subframe according to a UE using a semi-static scheme via higher layer signaling. A UE forwards CSI to a base station via an uplink control channel (PUCCH) according to a determined period in consideration of a transmission mode. If uplink data exists at the same time in a subframe in which CSI is transmitted, the CSI is transmitted via an uplink data channel (PUSCH) together with the data. The base station transmits transmission timing information appropriate for a UE to the UE in consideration of a channel status of each UE, a UE distribution status in a cell, and the like. The transmission timing information includes a period for transmitting CSI, offset, and the like and can be transmitted to each UE via an RRC message.

LTE system includes 4 types of CQI reporting mode. Specifically, the CQI reporting mode is divided into WB CQI and SB CQI according to a CQI feedback type and is divided into no PMI and single PMI depending on whether PMI is transmitted or not. In order to periodically report CQI, each UE receives information consisting of a combination of a period and an offset via RRC signaling.

CSI reporting types defined in LTE release-10 are described in the following.

A type 1 report supports CQI feedback for a UE on a selected subband. A type 1a report supports subband CQI and second PMI feedback. A type 2, a type 2b, and a type 2c reports support wideband CQI and PMI feedback. A type 2a report supports wideband PMI feedback. A type 3 report supports RI feedback. A type 4 report supports wideband CQI. A type 5 report supports RI and wideband PMI feedback. A type 6 report supports RI and PTI (precoding type indicator) feedback.

Massive MIMO

A recent wireless communication system considers introducing an active antenna system (hereinafter, AAS). Unlike a legacy passive antenna system that an amplifier capable of adjusting a phase and a size of a signal is separated from an antenna, the AAS corresponds to a system that each antenna is configured as an active antenna including such an active circuit as an amplifier. Since the AAS uses an active antenna, it is not necessary for the AAS to have a separate cable for connecting an amplifier with an antenna, a connector, other hardware, and the like. Hence, the AAS has characteristics that efficiency is high in terms of energy and management cost. In particular, since the AAS supports an electronic beam control scheme according to each antenna, the AAS enables an evolved MIMO technique such as forming a delicate beam pattern in consideration of a beam direction and a beam width, forming a 3D beam pattern, and the like.

As the evolved antenna system such as the AAS and the like is introduced, a massive MIMO structure including a plurality of input/output antennas and multi-dimensional antenna structure are also considered. As an example, in case of forming a 2D antenna array instead of a legacy straight antenna array, it may be able to form a 3D beam pattern by the active antenna of the AAS.

FIG. 6 illustrates a 2D active antenna system having 64 antenna elements.

Referring to FIG. 6, it is able to see that N_(t)=N_(v)·N_(h) number of antennas forms a shape of square. In particular, N_(h) and N_(v) indicate the number of antenna columns in horizontal direction and the number of antenna rows in vertical direction, respectively.

If the 3D beam pattern is utilized in the aspect of a transmission antenna, it may be able to perform semi-static or dynamic beam forming not only in horizontal direction but also in vertical direction of a beam. As an example, it may consider such an application as sector forming in vertical direction and the like. In the aspect of a reception antenna, when a reception beam is formed using massive antennas, it may be able to expect a signal power increasing effect according to an antenna array gain. Hence, in case of uplink, an eNB is able to receive a signal transmitted from a UE through a plurality of antennas. In this case, in order to reduce interference impact, the UE can configure transmit power of the UE to be very low in consideration of a gain of massive reception antennas.

FIG. 7 illustrates a 3D-MIMO system utilizing 2D-AAS. In particular, FIG. 7 shows a system that an eNB or a UE has a plurality of transmission/reception antennas capable of forming an AAS-based 3D beam.

Meanwhile, an antenna port corresponds to a concept of a logical antenna and does not mean an actual antenna element. Hence, the antenna port and the antenna element itself can be referred to as a virtual antenna and a physical antenna, respectively. A scheme of mapping an antenna port to a physical antenna element is an important element in designing the overall MIMO system. One-to-one mapping for mapping an antenna port to an antenna element and one-to-many mapping for mapping an antenna port to a plurality of antenna elements can be considered as the antenna mapping scheme.

Mapping an antenna port to an antenna element is represented as a virtualization matrix B in equation 11. In this case, x corresponds to a signal transmitted from the antenna port and z corresponds to a signal transmitted from the antenna element. The number of antenna ports can be smaller than the number of antenna elements. Yet, for clarity, assume that the number of antenna ports also corresponds to N. b_(n) corresponds to a virtualization vector indicating a relation that an n^(th) antenna port is mapped to antenna elements. If the number of non-zero element of the virtualization vector b_(n) corresponds to 1, it indicates the one-to-one mapping scheme. If the number of non-zero element of the virtualization vector b_(n) corresponds to a plural number, it indicates the one-to-many mapping scheme. z=Bx=└b ₀ b ₁ . . . b_(N) _(t) ⁻¹ ┘x.   [Equation 11]

In equation 11, in order to consider that signal energy of an antenna port and signal energy of an antenna element are the same, assume that a virtualization vector is normalized to ∥b_(n)∥1. In the following, a relation between an antenna element and an antenna port is explained in more detail with reference to the drawing.

FIG. 8 illustrates a relation between an antenna element and an antenna port in a 2D AAS system to which massive MIMO is applied. In particular, the left drawing of FIG. 8 shows 32 antenna elements in total, i.e., 32 physical antennas, and the right drawing of FIG. 8 shows 32 antenna ports in total, i.e., 32 logical antennas.

In particular, FIG. 8 shows a grouping scheme of antenna elements and a grouping scheme of antenna ports. FIG. 8 also shows mapping between an antenna element and an antenna port. Referring to FIG. 8, it is able to see that antenna elements are grouped as antenna columns in vertical direction. Specifically, the antenna elements are divided into 4 groups including E(0), E(1), E(2), and E(3). And, the 32 antenna ports are also divided into 4 groups to form groups including F(0), F(1), F(2), and F(3).

In this case, antenna ports belonging to a group F(i) are virtualized using all antenna elements belonging to a group E(i). A virtualization vector of each antenna port belonging to the group F(i) is differently configured. One antenna port is selected from each antenna port group to form a group T(i). Each antenna port belonging to the group T(i) uses an identical virtualization vector to be mapped to a different antenna element group. An RS for each antenna port belonging to the group T(i) is transmitted to an identical OFDM symbol.

In a FD MIMO system, an eNB can set a plurality of CSI-RS resources to a UE in a single CSI process. In this case, the CSI process corresponds to an operation of making a feedback with an independent feedback configuration. For example, 8 CSI-RS resources can be configured within a single CSI process. In this case, 8 CSI-RSs correspond to a group T(0) to a group T(7) in FIG. 8.

Since different virtualization is applied, different beamforming is applied to each of 8 4-port CSI-RSs . For example, in case of a CSI-RS corresponding to the T(0), vertical direction beamforming is applied to the CSI-RS using a zenith angle of 100 degrees and the CS-RS is configured with a zenith angle difference of 5 degrees. Hence, vertical direction beamforming is applied to a CSI-RS corresponding to the T(7) with a zenith angle of 135 degrees.

The UE selects a CSI-RS of a strong channel from among 8 CSI-RSs, calculates CSI on the basis of the selected CSI-RS, and report the CSI to the eNB. In this case, the UE additionally reports the selected CSI-RS to the eNB through a beam index (BI) value. For example, if a channel of a first CSI-RS corresponding to T(0) is strongest, the UE sets the BI to 0 to report the channel to the eNB. In this case, since the beam index corresponds to information on the selected CSI-RS, the beam index can also be referred to as a CRI (CSI-RS resource indicator).

In the following, for clarity, assume that a BI indicates selection of a CSI-RS. More specifically, the BI may indicate a combination between a specific CSI-RS and a specific antenna port. For example, the BI selects a CSI-RS from among 8 CSI-RSs included in a CSI process and additionally selects a combination between an antenna port 15 and an antenna port 16 from the selected CSI-RS. If it is able to select one from among a combination between an antenna port 15 and an antenna port 16 and a combination between an antenna port 17 and an antenna port 18 in each CSI-RS, the BI is able to indicate one of 16 values. In particular, a BI for a combination between an antenna port 15 and an antenna port 16 of a first CSI-RS is mapped in an order of 0, a BI for a combination between an antenna port 17 and an antenna port 18 of the first CSI-RS is mapped in an order of 1, a BI for a combination between an antenna port 15 and an antenna port 16 of a second CSI-RS is mapped in an order of 2, and a BI for a combination between an antenna port 17 and an antenna port 18 of the second CSI-RS is mapped in an order of 3. In particular, a BI for a combination between an antenna port 17 and an antenna port 18 of an eighth CSI-RS is lastly mapped in an order of 15. In particular, when a BI indicates a combination between antenna ports, the proposed scheme of the present invention can be applied as it is.

When a CSI is periodically transmitted via PUCCH, if a BI is collided with other CSI values in the same subframe, it is necessary to define a dropping rule based on a priority. According to a current LTE standard document, reporting types shown in table 1 have been defined for PUCCH CSI feedback.

TABLE 1 Type 1 report supports CQI feedback fix the UE selected sub-bands Type 1a report supports subband CQI and second PMI feedback Type 2, Type 2b, and Type 2c report supports wideband CQI and PMI feedback Type 2a report supports wideband PMI feedback Type 3 report supports RI feedback Type 4 report supports wideband CQI Type 5 report supports RI and wideband PMI feedback Type 6 report supports RI and PTI feedback

In some cases, a plurality of reporting types are configured to be fed back in the same subframe. This can be represented as a collision occurs. In this case, a UE feedbacks a single reporting type only and drops the remaining reporting types.

If a UE feedbacks a BI, reporting types shown in table 2 can be newly configured.

TABLE 2 Type 7 report supports BI feedback Type 8 report supports BI and RI feedback Type 9 report supports BI and RI and PTI feedback Type 10 report supports BI and RI and wideband PMI feedback Type 11 report supports BI and wideband PMI feedback

In the new reporting types shown in table 2, a BI may correspond to a long-term BI or a short-term BI. As the new reporting types are defined, since a BI corresponds to an important value that influences on calculating PMI, RI, and CQI, it is preferable to transmit the BI with a high priority. In particular, all reporting types capable of transmitting the BI have a priority identical to a priority of legacy reporting types 3, 5, and 6.

FIG. 9 illustrates an example of PUCCH CSI feedback according to an embodiment of the present invention. Referring to FIG. 9, it is able to see that an RI is configured by a reporting interval of 60 ms and an offset of 0 ms and PMI/CQI is configured by a reporting interval of 5 ms and an offset of 5 ms.

A BI has an offset identical to PMI/CQI and an interval of the BI is defined by N multiple of an interval of the PMI/CQI. (In this case, an eNB informs a UE of the N via higher layer signaling such as RRC signaling.) As a result, the BI is collided with the PMI/CQI. In this case, as mentioned in the foregoing description, since a priority of the BI is higher than a priority of the PMI/CQI, the BI is reported while the PMI/CQI is dropped.

In addition, when the RI shown in FIG. 9 is reported, a long-term BI can be transmitted together. When the BI shown in FIG. 9 is transmitted, a wideband PMI (i.e., PMI corresponding to W1 in a dual codebook structure consisting of W1 and W2) can be transmitted together.

Meanwhile, if the number of selected CSI-RS ports changes according to a BI value, a maximum rank value is changed. For example, if MIMO capability of a UE, i.e., the number of layers capable of being maximally supported, corresponds to 8, it may consider a case that BIs are defined as table 3 described in the following.

A maximum rank shown in table 3 is determined by the number of CSI-RS ports selected from BI and a minimum value of the MIMO capability of the UE. In this case, a bit size (bit width) of a BI may vary according to an RI value transmitted in a subframe #0. For example, if an RI corresponds to 8, since available BI values correspond to 0 and 1 only, a BI has a bit size of 1. If an RI corresponds to 4, since available BI values correspond to 0 to 3 only, a BI has a bit size of 2. If an RI corresponds to 2, since all BI values are available, a BI has a bit size of 3.

TABLE 3 Number of selected Maximum BI CSI-RS ports rank 0 8 8 1 8 8 2 4 4 3 4 4 4 2 2 5 2 2 6 2 2 7 2 2

Referring to FIG. 9, if an RI selected in a subframe #0 corresponds to 2, a bit size (bit width) of a BI is calculated by 3 bits until a next RI is reported (i.e., a subframe #5, a subframe #25, and a subframe #45). Yet, since a new RI is configured by 4 in a subframe #60, a bit size of a following BI is calculated by 2 bits.

As mentioned in the foregoing description, if a bit size of a BI varies, it may increase a coding gain, thereby increasing reliability of BI information transmission. If the bit size of the BI does not vary, a BI value is reported by a bit size of 3 bits corresponding to all BI values irrespective of an RI value. In particular, the bit size of the BI is determined by the number of selectable beams defined in a CSI process set to a UE. If a unit of a resource selected by a BI corresponds to a CSI-RS resource, a bit size of the BI is defined by a rounded value of a log₂(N) value when the N number of CSI-RSs are configured in a process. If a unit of a resource selected by a BI corresponds to a CSI-RS port, a bit size of the BI is defined by a rounded value of a log₂(N) value when the N numbers of CSI-RS ports is selectable in a process.

Meanwhile, since the number of ports varies according to a BI value, a codebook to be used by a UE is changed. In particular, if the number of CSI-RS ports selected by a BI corresponds to N, it is necessary to report PMI using N Tx codebook. For example, if a BI corresponds to 0 in a subframe #5, 8 Tx codebook is used. If a BI corresponds to 3, 4 Tx codebook is used. Similarly, when a BI and W1 are transmitted together, the W1 is defined by PMI of a different codebook according to a value of the BI. Moreover, if a codebook is changed due to a BI, a PMI payload size and a PMI subsampling scheme are changed as well.

FIG. 10 is a diagram illustrating a change of a bit size of an RI according to a BI in accordance with an embodiment of the present invention. In particular, in FIG. 10, assume a case that a reporting interval of a BI is configured to be equal to or greater than an interval of an RI.

Referring to FIG. 10, as shown in table 3, when the number of CSI-RS ports varies according to 8 BI values in total, a bit size of an RI is determined by the number of CSI-RS ports selected by a BI and a minimum value of MIMI capability of a UE. When a value of the MIMO capability corresponds to 8, if a BI selected in a subframe #0 corresponds to 3, a bit size of an RI is calculated by log₂(min(MIMO capability (=8), CSI-RS port number (=4)))=2 until a next BI is reported (i.e., in a subframe #5, a subframe #25, and a subframe #45). If a new BI is configured in a subframe #60, a bit size of an RI, which is reported after the subframe #60, may have a new value.

In particular, in a legacy 3GPP 36.212 standard document described in table 4, it is preferable to change a part described as ‘the configured number of CSI-RS ports’ in a corresponding CSI process used for determining a bit size of an RI with ‘the configured number of CSI-RS ports indicated by a BI’ in the CSI process.

TABLE 4 For rank indication (RI) (RI only, joint report of RI and i1, and joint report of RI and PTI)  The corresponding bit widths for RI feedback for PDSCH transmissions are given by Tables 5.2.2.6.1-2,  5.2.2.6.2-3, 5.2.2.6.3-3, 5.2.3.3.1-3, 5.2.3.3.1-3A, 5.2.3.3.2-4, and 5.2.3.3.2-4A, which are determined  assuming the maximum number of layers as follows:   If the maxLayersMIMO-r10 is configured for the DL cell, the maximum number of layers is   determined according to maxLayersMIMO-r10 for the DL cell.   Else,     If the UE is configured with transmission mode 9, and the supportedMIMO-CapabilityDL-     r10 field is included in the UE-EUTRA-Capability, the maximum number of layers is     determined according to the mininium of the configured number of CSI-RS ports and the     maximum of the reported UE downlink MIMO capabilities for the same band in the     corresponding band combination.     If the UE is configured with transmission mode 9, and the supportedMIMO-CapabilityDL-     r10 field is not included in the UE-EUTRA-Capability, the maximum number of layers is     determined according to the minimum of the configured number of CSI-RS ports and ue-     Category (without suffix).     If the UE is configured with transmission mode 10, and the supportedMIMO-CapabilityDL-     r10 field is included in the UE-EUTRA-Capability, the maximum number of layers for each     CSI process is determined according to the minimum of the configured number of CSI-RS     ports for that CSI process and the maximum of the reported UE downlink MIMO capabilities     for the same band in the corresponding band combination,     If the UE is configured with transmission mode 10, and the supportedMIMO-CapabilityDL-     r10 field is riot included in the UE-EUTRA-Capability, the maximum number of layers for     each CSI process is determined according to the minimum of the configured at-timber of CSI-     RS ports for that CSI process and ue-Category (without suffix).     Otherwise the maximum number of layers is determined according to the minimum of the     number of PBCH antenna ports and ue-Category (without suffix).

Tables 5 to 11 in the following correspond to table 5.2.2.6.1-2, table 5.2.2.6.2-3, table 5.2.2.6.3-3, table 5.2.3.3.1-3, table 5.2.3.3.1-3A, table 5.2.3.3.2-4, and table 5.2.3.3.2-4A of 3GPP 36.212 standard document.

Specifically, tables 5 and 8 define a bit size of a rank indicator for performing wideband CQI report according to the number of CSI-RS ports. Table 6, table 7 and table 10 define a bit size of a rank indicator for performing subband CQI report according to the number of CSI-RS ports. Table 9 defines a bit size of a rank indicator according to the number of CSI-RS ports when a rank indicator and W1 are reported in a manner of being combined. Similarly, table 11 defines a bit size of a rank indicator according to the number of CSI-RS ports when a rank indicator and PTI (precoder type indication) are reported in a manner of being combined.

Referring to tables 5 to 11, when a bit size of a rank indicator is determined, the number of CSI-RS ports is used. Meanwhile, the present invention proposes a method of applying the configured number of CSI-RS ports indicated by a BI in a corresponding CSI process.

TABLE 5 Table 5.2.2.6.1-2 Bit width 4 antenna ports 8/12/16 antenna ports Max Max 2 antenna 1 or 2 Max 4 1 or 2 Max 4 Max 8 Field ports layers layers layers layers layers Rank 1 1 2 1 2 3 indication

TABLE 6 Table 5.2.2.6.2-3 Bit width 4 antenna ports 8/12/16 antenna ports Max Max 2 antenna 1 or 2 Max 4 1 or 2 Max 4 Max 8 Field ports layers layers layers layers layers Rank 1 1 2 1 2 3 indication

TABLE 7 Table 5.2.2.6.3-3 Bit width 4 antenna ports 8/12/16 antenna ports Max Max 2 antenna 1 or 2 Max 4 1 or 2 Max 4 Max 8 Field ports layers layers layers layers layers Rank 1 1 2 1 2 3 indication

TABLE 8 Table 5.2.3.3.1-3 Bit width 4 antenna ports 8/12/16 antenna ports Max Max 2 antenna 1 or 2 Max 4 1 or 2 Max 4 Max 8 Field ports layers layers layers layers layers Rank 1 1 2 1 2 3 indication

TABLE 9 Table 5.2.3.3.1-3A Bit width 4 antenna ports 8 antenna ports Max Max 2 antenna 1 or 2 Max 4 1 or 2 Max 4 Max 8 Field ports layers layers layers layers layers Rank 1 1 2 4 5 5 indication il — — —

TABLE 10 Table 5.2.3.3.2-4 Bit width 4 antenna ports 8 antenna ports Max Max 2 antenna 1 or 2 Max 4 1 or 2 Max 4 Max 8 Field ports layers layers layers layers layers Rank 1 1 2 1 2 3 indication

TABLE 11 Table 5.2.3.3.2-4A Bit width 4 antenna ports 8/12/16 antenna ports Max Max 2 antenna 1 or 2 Max 4 1 or 2 Max 4 Max 8 Field ports layers layers layers layers layers Rank indication 1 1 2 1 2 3 Precoder type — — — 1 1 1 indication

If a bit size of an RI is determined according to a BI value, it is able to increase a coding gain of the RI, thereby increasing reliability of RI information transmission.

Yet, if a bit size of an RI is differently configured according to a BI value, system complexity may increase. Hence, in the legacy 3GPP 36.212 standard document shown in table 4, it may change ‘the configured number of CSI-RS ports’ in a corresponding CSI process used for determining a bit size of an RI with ‘the maximum configured number of CSI-RS ports capable of being indicated by a BI’ to configure the bit size of the RI. For example, in table 3, since the maximum number of ports capable of being selected by a BI corresponds to 8, an RI bit size is determined using such a value as 8 and a value of MIMO capability.

Similarly, referring to tables 5 to 11, when a bit size of a rank indicator is determined, the number of CSI-RS ports is used in a corresponding CSI process. Meanwhile, according to the present invention, it may apply the maximum configured number of CSI-RS ports capable of being indicated by a BI in the CSI process.

When a BI and an RI are reported together via PUCCH, a UE can report the BI and the RI using reporting types 8, 9, and 10. In this case, ‘the configured number of CSI-RS ports’ can also be changed with ‘the maximum configured number of CSI-RS ports capable of being indicated by a BI’ to configure a bit size of an RI.

Meanwhile, the aforementioned method of determining a bit size can also be applied to a report of RI only, a combination report of RI and W1 (or, i1), a combination report of RI and BI (or, CRI), a combination report of RI and PTI, and the like to determine a bit size based on ‘the maximum configured number of CSI-RS ports capable of being indicated by a BI’. Of course, the method can also be applied to a combination report of RI, BI (or, CRI), and W1 (or, i1) and a combination report of RI, BI (or, CRI), and PTI to determine a bit size based on ‘the maximum configured number of CSI-RS ports capable of being indicated by a BI’.

In addition, although table 4 describes transmission modes (TM) 9 and 10, if a new TM (e.g., TM 11) is defined for 3D MIMO or FD MIMO, the TM 10 shown in table 4 is modified into TM 11 and, as mentioned in the foregoing description, a bit size of an RI can be determined by changing a part described as ‘the configured number of CSI-RS ports’ in a corresponding CSI process used for determining the bit size of the RI with ‘the configured number of CSI-RS ports indicated by a BI’ in the CSI process or ‘the maximum configured number of CSI-RS ports capable of being selected by a BI’.

FIG. 11 is a flowchart illustrating a method of determining a bit size of a rank indicator according to an embodiment of the present invention.

Referring to FIG. 11, a UE configures a plurality of CSI-RS (channel status information—reference signal) resources for a single CSI (channel status information) process via a higher layer in the step S1101. In this case, independent precoding is applied to each of a plurality of the CSI-RS resources.

Subsequently, the UE measures a channel by receiving a CSI-RS via the CSI-RS resources and selects a CSI-RS resource of the best quality in the step S1103. Subsequently, the UE determines a rank based on the CSI-RS and reports a CRI indicating the selected CSI-RS resource to an eNB together with a rank indicator in the step S1105.

In particular, a bit size of the rank indicator is determined based on the maximum number of antenna ports for a plurality of the CSI-RS resources. In addition, it may also consider the maximum number of layers capable of being supported by the UE. In this case, the UE can provide the eNB with information on the maximum number of layers capable of being supported by the UE in advance.

FIG. 12 is a diagram for a base station and a user equipment capable of being applied to an embodiment of the present invention.

If a relay is included in a wireless communication system, communication is performed between a base station and the relay in backhaul link and communication is performed between the relay and a user equipment in access link. Hence, the base station and the user equipment shown in the drawing can be replaced with the relay in accordance with a situation.

Referring to FIG. 12, a wireless communication system includes a base station (BS) 1210 and a user equipment (UE) 1220. The BS 1210 includes a processor 1213, a memory 1214 and a radio frequency (RF) units 1211/1212. The processor 1213 can be configured to implement the proposed functions, processes and/or methods. The memory 1214 is connected with the processor 1213 and then stores various kinds of information associated with an operation of the processor 1213. The RF units 1211/1212 are connected with the processor 1213 and transmits and/or receives a radio signal. The user equipment 1220 includes a processor 1223, a memory 1224 and a radio frequency (RF) unit 1221/1222. The processor 1223 can be configured to implement the proposed functions, processes and/or methods. The memory 1224 is connected with the processor 1223 and then stores various kinds of information associated with an operation of the processor 1223. The RF unit 1221/1222 is connected with the processor 1223 and transmits and/or receives a radio signal. The base station 1210 and/or the user equipment 1220 may have a single antenna or multiple antennas.

The above-described embodiments correspond to combinations of elements and features of the present invention in prescribed forms. And, the respective elements or features may be considered as selective unless they are explicitly mentioned. Each of the elements or features can be implemented in a form failing to be combined with other elements or features. Moreover, it is able to implement an embodiment of the present invention by combining elements and/or features together in part. A sequence of operations explained for each embodiment of the present invention can be modified. Some configurations or features of one embodiment can be included in another embodiment or can be substituted for corresponding configurations or features of another embodiment. And, it is apparently understandable that an embodiment is configured by combining claims failing to have relation of explicit citation in the appended claims together or can be included as new claims by amendment after filing an application.

In this disclosure, a specific operation explained as performed by an eNode B may be performed by an upper node of the eNode B in some cases. In particular, in a network constructed with a plurality of network nodes including an eNode B, it is apparent that various operations performed for communication with a user equipment can be performed by an eNode B or other networks except the eNode B. ‘eNode B (eNB)’ may be substituted with such a terminology as a fixed station, a Node B, a base station (BS), an access point (AP) and the like.

Embodiments of the present invention can be implemented using various means. For instance, embodiments of the present invention can be implemented using hardware, firmware, software and/or any combinations thereof. In the implementation by hardware, a method according to each embodiment of the present invention can be implemented by at least one selected from the group consisting of ASICs (application specific integrated circuits), DSPs (digital signal processors), DSPDs (digital signal processing devices), PLDs (programmable logic devices), FPGAs (field programmable gate arrays), processor, controller, microcontroller, microprocessor and the like.

In case of the implementation by firmware or software, a method according to each embodiment of the present invention can be implemented by modules, procedures, and/or functions for performing the above-explained functions or operations. Software code is stored in a memory unit and is then drivable by a processor.

The memory unit is provided within or outside the processor to exchange data with the processor through the various means known in public.

Detailed explanation on the preferred embodiment of the present invention disclosed as mentioned in the foregoing description is provided for those in the art to implement and execute the present invention. While the present invention has been described and illustrated herein with reference to the preferred embodiments thereof, it will be apparent to those skilled in the art that various modifications and variations can be made therein without departing from the spirit and scope of the invention. For instance, those skilled in the art can use each component described in the aforementioned embodiments in a manner of combining it with each other. Hence, the present invention may be non-limited to the aforementioned embodiments of the present invention and intends to provide a scope matched with principles and new characteristics disclosed in the present invention.

While the present invention has been described and illustrated herein with reference to the preferred embodiments thereof, it will be apparent to those skilled in the art that various modifications and variations can be made therein without departing from the spirit and scope of the invention. Thus, it is intended that the present invention covers the modifications and variations of this invention that come within the scope of the appended claims and their equivalents. And, it is apparently understandable that an embodiment is configured by combining claims failing to have relation of explicit citation in the appended claims together or can be included as new claims by amendment after filing an application.

INDUSTRIAL APPLICABILITY

The present invention can be used for a wireless communication device such as a terminal, a relay, a base station and the like. 

What is claimed is:
 1. A method of reporting a rank indicator, which is reported to a base station by a user equipment, UE, in a wireless access system, comprising the steps of: receiving, from the base station, configuration information for a plurality of channel state information-reference signal, CSI-RS, resources for a single channel state information, CSI, process via a higher layer; selecting a single CSI-RS resource from the plurality of CSI-RS resources; determining an indicator corresponding to the selected single CSI-RS resource from a set of indicator values supported for the single channel CSI process; and reporting the determined indicator and a rank indicator in same reporting instance to the base station, wherein a bit size of the rank indicator is determined based on a maximum number of layers the UE is capable of supporting and a number of antenna ports corresponding to the determined indicator.
 2. The method of claim 1, further comprising the step of transmitting information on the maximum number of layers the UE is capable of supporting to the base station.
 3. The method of claim 1, further comprising the step of determining the rank indicator using the selected single CSI-RS resource.
 4. The method of claim 1, wherein independent precoding is applied to each of the plurality of CSI-RS resources.
 5. The method of claim 1, wherein the indicator corresponding to the selected single CSI-RS resource and the rank indicator are reported, through a physical uplink control channel, PUCCH, or a physical uplink shared channel, PUSCH, to the base station.
 6. The method of claim 1, wherein the indicator corresponding to the selected single CSI-RS resource and the rank indicator are reported to the base station periodically.
 7. A user equipment, UE, in a wireless access system, comprising: a transmitter and a receiver; and a processor operatively connected to the transmitter and the receiver, wherein the processor configured to: receive, from a base station, configuration information for plurality of channel state information-reference signal, CSI-RS, resources for a single channel state information, CSI, process via a higher layer, determine an indicator corresponding to select a single CSI-RS resource from among the plurality of CSI-RS resources for a channel status information, CSI, process, select a single CSI-RS resource from the plurality of CSI-RS resources, report the determined indicator and a rank indicator in a same reporting instance to the base station. wherein a bit size of the rank indicator is determined based on a maximum number of layers the UE is capable of supporting and a number of antenna ports corresponding to the determined indicator.
 8. The UE of claim 7, wherein the processor is further configured to transmit information on the maximum number of layers the UE is capable of supporting to the base station.
 9. The UE of claim 7, wherein the processor is further configured to determine the rank indicator using the selected single CSI-RS resource.
 10. The UE of claim 7, wherein independent precoding is applied to each of the plurality of CSI-RS resources.
 11. The UE of claim 6, wherein the indicator corresponding to the selected single CSI-RS resource and the rank indicator are reported, through a physical uplink control channel, PUCCH, or a physical uplink shared channel, PUSCH, to the base station.
 12. The UE of claim 6, wherein the indicator corresponding to the selected single CSI-RS resource and the rank indicator are reported to the base station periodically. 